Novel fluorinated liquid crystals. II. The synthesis and phase transitions of a novel type of ferroelectric liquid crystals containing 1,4-tetrafluorophenylene moiety

4-[(S)-2-Methylbutoxycarbonyl]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl]benzoates have been prepared from the starting material 1-pentafluorophenyl-2-trimethylsilylacetylene. Polarizing microscope textural observation and DSC measurements of the phase transitions of these novel compounds showed that they were liquid crystals with chiral smectic C phase (S*_C), smectic A(S_A) and cholesteric (Ch) phases. The effects of the alkoxy chain length on the transition temperatures and enthalpies were also studied.     

TGBA phase in a series of propionates containing two stereocenters

A homologous series of chiral materials, (R)-2-pentyl (S)-2-(6-(4-(4'-alkoxyphenyl)benzoyloxy)-2-naphthyl)propionates (R,S)PmPBNP (m = 7-14), derived from a naphthalene ring as part of the core structure in conjunction with a chiral tail containing two stereocentres has been synthesized for the investigation. The twist grain boundary smectic A (TGB_A) and smectic C (TGB*_C) phases were characterized by the microscopic textures of compounds packed in two untreated glass slides, Cano wedge cell, or homogeneously aligned cell. It was found that the occurrence of these phases depend remarkably on the nature of alkyl chain length m; in the case of shorter alkyl chain length (m = 7-10), the TGB_A phase behaves as a metastable-like phase mediated between N and S_A phases in a short temperature range, whereas in the longer chain length (m' 11-14), both the TGB_A and TGB_C phases become thermodynamically stable phases with a wide temperature range. Consequently, this series of chiral materials resulted in two different mesophase sequences: N-TGB_A-S_A-S_Cand N-TGB_A-TGB_C. A kind of parquet texture displaying two types of domain with different relative directions of the smectic layer normal was found in the S phase from the materials (m = 9-14) packed in 2 μm homogeneously aligned cells and cooled down from the isotropic liquid without applying an electric field. The magnitudes of spontaneous polarization (P_s) in the S_C and TGB_C phases showed that the P_s values are nearly the same for all compounds at the same temperature below the Curie point. Dielectric measurements revealed no significant occurrence of soft mode switching in the TGB_A phase.     

Photomechanically induced ferroelectricity in smectic liquid crystals

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm^-2) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P_s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P_s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm^-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P_s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.     

13C N.M.R. and 14N N.Q.R. in ferroelectric liquid crystals Polar versus quadrupolar ordering

¹³C nuclear magnetic resonance and ¹⁴N nuclear quadrupole resonance spectra of ferroelectric smectic C*liquid crystals and their non-chiral analogues allow for a microscopic determination of the polar and quadrupolar (or bipolar) biasing of rotation around the long molecular axis as well as for a determination of the anisotropy in the fluctuations of this axis. The results show that the microscopic origin of the biquadratic coupling between the polarization and the tilt, which has been recently introduced into the extended Landau model of the S_A-S*_C transition, is the quadrupolar (or bipolar) rotational bias induced by the anisotropy in the fluctuations of the long molecular axis. The tilt induced anisotropy in the fluctuations is practically identical in chiral and non-chiral smectic C phases.     

Electroclinic effect near a smectic A-chiral smectic I transition

The electroclinic effect in the S_A phase of (S) (4'-decyloxy-2'-hydroxybenzylidene)-4-amino(1-ethoxycarbonylethyl) cinnamate has been studied. The induced tilt angles have been measured as a function of temperature near the S_A-S*_I transition at 87°C. The response of the material to different voltages and frequencies has been analysed by using two complementary optical techniques. At low A.C. probe fields the amplitude of the induced tilt shows a relaxational behaviour which can be described by a single relaxation time. The relaxation frequencies have been found to be linear in temperature near the transition, in a similar way to the S_A-S*_C transition.     

Dielectric and optical studies near the chiral smectic C-smectic A transition of (S)-2-hydroxy-4-decyloxybenzylidene-4'-amino-2p'-methylbutylcinnamate

The complex dielectric permittivity of HDOBAMBC has been measured in the vicinity of the chiral smectic C-smectic A transition as a function of temperature and frequency. The Goldstone mode contribution has been clearly identified and its relaxation frequency has been determined over the whole S*_C range. High accuracy tilt angle measurements have also been performed by using a new method which allows its determination in a direct and virtually continuous way. These experimental results together with additional measurements of the polarization and helical pitch have been compared with those of DOBAMBC and analized in the framework of the generalized Landau theory. As for DOBAMBC a reentrant behaviour S*_C-S_C-S*_C has been observed under an electric field near the S*_C-S_A transition.     

A new type of layer structure defects in chiral smectics

Under a low frequency AC electric field applied to the S_A phase an apparently homogeneous planar structure is obtained which is in fact composed of a system of parallel stripes perpendicular to the smectic layers. This texture persists even in the S*_C phase. The stripes are explained as a contrast on elongated focal conics originating from undulations of smectic layers. Such undulations are formed as a consequence of a reduction in the smectic layer thickness due to the electroclinic effect. The deformation of smectic layers has been calculated on the basis of the elastic free energy associated with their deformation.     

Synthesis and characterization of new ferroelectric liquid crystals containing a (2S)-2-[6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'- naphthyl] propionate mesogenic group and oligo(oxyethylene) spacers

Three series of ferroelectric liquid crystals containing a (2S)-2-(6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'-naphthyl)propionate mesogenic group and oligo(oxyethylene) spacers were synthesized. These obtained liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC), optical polarized microscopy (POM), and X-ray powder diffraction measurements. Some of these materials containing four phenyl rings of ester cores (i.e. -Ph-Ph-COO-naph-) and chiral heptyl tail exhibited a rich mesomorphic behaviour, a blue phase (BP), a cholesteric phase (Ch), a smectic A (S_A), a twist grain boundary A (TGB_A), and a chiral smectic C (S_C*) phase. Another series containing four phenyl rings of ester cores and chiral butyl and pentyl chain tails revealed only a S_A phase and a S_C* phase. Moreover, a crystal E phase was observed in the short spacer chain (n = 0 or 1) homologues of three series of compounds. Also, the mesomorphism properties were discussed as a function of spacer units, numbers of aromatic rings of core, and different terminal asymmetric moieties.     

The characteristic dielectric behaviour in ferroelectric liquid crystals at a phase transition and the contribution of the soft mode

The characteristic dielectric behaviour of ferroelectric liquid crystals with a large spontaneous polarization has been studied as functions of the D.C. bias field, frequency, cell thickness and applied pressure. Under the condition in which the contribution of the Goldstone mode is suppressed, a sharp peak in the temperature dependence of the dielectric constant is clearly observed at the transition between S_A and S*_C phases T_SC*sA. The relaxation of the soft mode is observed both in the S_A and S*_C phases by eliminating the contribution of the Goldstone mode under a D.C. bias field. Another relaxation is also observed in the S*_C phase around several kHz in addition to that of the soft mode and the Goldstone mode. The pressure effect on the soft mode was also studied.     

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

Proton spin-lattice relaxation studies were carried out in the S_A and S*_C phases of the liquid crystal CI IPNOC using both conventional and fast field cycling NMR techniques. T₁ dispersion curves were obtained at two different temperatures for each mesophase covering frequencies from 10² to 3 × 10⁸ Hz. In both mesophases the T₁ data can be described assuming the presence of three different relaxation mechanisms, namely local molecular rotations, molecular self-diffusion and collective motions. The self-diffusion constant D₁ was evaluated for several temperatures and the activation energy associated with the diffusion process was obtained. The expected contribution of the soft-mode for the spin-lattice relaxation could not be separated from the contribution of other collective motions. The correlation times associated with the rotations around the molecular long axis and with the fluctuations of this axis were evaluated for both the S_A and the S*_C phases.     

Behaviour of the pitch of the cholesteric and chiral smectic C helix near and at the cholesteric-smectic A-chiral smectic C multicritical point

A cholesteric-smectic A-chiral smectic C multicritical point was established in binary mixtures of 4-n-hexyloxyphenyl-4'-(2'-methylbutyl)biphenyl-4-carboxylate with cholesteryl myristate or cholesteryl benzoate. At this point the cholesteric phase, smectic A phase, and chiral smectic C phase become indistinguishable. Whereas the pitch of the cholesteric helix at the S_A-Ch phase transition is infinite already in the vicinity of the multicritical point, the pitch of the cholesteric helix at the S*_C-Ch transition becomes infinite only at this point. In accord with the theory of Beresnev the pitch of the chiral smectic C helix remains finite at the multicritical point. Additional high pressure experiments support the results obtained at atmospheric pressure.     

Synthesis and characterization of smectic polymesomorphism in higher homologues of nO.m compounds

Some of the higher homologues of the N-(p-n-alkoxybenzylidene)p-n-alkylanilines, n O.m, compounds with n ≧ 8 and m ≧ 5 have been synthesized and the smectic polymesomorphism studied. The transition temperatures were obtained from both thermal microscopy and differential scanning calorimetry. These compounds in the n O.m series exhibit the usual phase sequences, viz. S_AS_CS_BS_G, and S_AS_BS_G apart from S_AS_CS_FS_G by 9O.8 (and some higher n O.m, like 9O.6). These phases were confirmed by miscibility studies with standard materials. The smectic F phase in 9O.8 compound is also confirmed by an X-ray diffraction study.     

The ferro-, ferri- and antiferro-electric properties of a series of novel 2- or 3-substituted-alkyl 4-(4'-dodecyloxybiphenyl-4-carbonyloxy)-benzoate esters

This paper reports the synthesis of six chiral liquid crystalline 2- or 3-substituted-alkyl 4-(4'-dodecyloxybiphenyl-4-carbonyloxy)benzoates, their mesophase assignments and the phase transition temperatures. All six materials display S*_A followed by S*_C phases on cooling from their isotropic liquids. The magnitudes of switched tilt angle, spontaneous polarization, helical twist sense (L.H.) and direction of polarization (P_s(-)) are given for the S*_C phases. Close inspection of the S*_C phases revealed that for the six compounds, two showed a ferrielectric (S*_Cγ) and an antiferroelectric (S*_A) subphase, two showed only a ferrielectric subphase and one showed only an antiferroelectric subphase. The field-dependent behaviour of the switched tilt angle of the ferrielectric and antiferroelectric phases of some of the compounds is also presented.     

Mesomorphic properties of a homologous series of chiral liquid crystals containing the α-chloroester group

The mesomorphic properties of the N*, S*_c and higher ordered smectic phases have been investigated for a homologous series of 4-[(S)-2-chlor-3-methylbutanoyloxy]-4'-(4-n-alkyloxy-benzoyloxy)biphenyls. They have been characterized by optical texture observation, differential scanning calorimetry (DSC), small angle X-ray scattering and electro-optic measurements. The compounds exhibit a strongly twisted cholesteric phase and smectic phases with large spontaneous polarization and tilt angle values. One or two higher ordered, monotropic smectic phases were found which significantly differ in their rotational viscosity.     

Synthesis and mesomorphic properties of compounds exhibiting the undulated twist grain boundary smectic C* phase

The synthesis and characterization of three homologous series of compounds exhibiting the undulated twist grain boundary smectic C* (UTGB_C*) phase are reported. The chiral mesophases have been obtained using cholesterol as the chiral moiety. Cholestanol and [S]-[+]-octan-2-ol have also been used as the chiral moiety for comparitive purposes. In addition to this novel phase, cholesteric, smectic A, smectic C* and TGB_A phases have also been observed. The mesophases were characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and measurement of helical pitch.     

Thickness dependence of chiral smectic C liquid crystal phase transitions

In this paper we consider the influence of solid boundaries on the transition temperatures of a chiral smectic C liquid crystal. Particular attention has been paid to the S*_C-S_A transition. A simple model to explain the thickness dependence of the S*_C-S_A transition is proposed. An experimental method to determine some elastic constants and the anchoring energy of ferroelectric liquid crystal molecules is demonstrated.     

New TGBA series exhibiting a SC*SASA*N* phase sequence

A new series with a chiral tolane core has been synthesized. These materials belong to the optically active series: 3-fluoro-4-[(R) or (S)-1-methylheptyloxy]4'-(4'-alkyloxy-3'-fluorobenzoyloxy)tolanes (nFBTFO₁M₇). For the first time, the helical S_A* phase or TGB_A phase is found in all of the derivatives from heptyloxy to octadecyloxy. The S_A-S_A*-N* phase sequence is observed in several compounds with short chains and the S_C*-S_A-S_A*-N* phase sequence is obtained with the decyloxy derivative. The TGB_A phase has filament or cholesteric textures. The helical pitch of the TGB_A phase is short for the octadecyloxy derivative and is compared with that for different chain lengths as a function of temperature. The layer spacing in the TGB_A phase shows that it is incommensurate with the fully elongated molecular length.     

Molecular dynamics of a ferroelectric smectogen in its smectic phases by means of 2H NMR spectroscopy

In this work the dynamic behaviour of the ferroelectric liquid crystal (-)-(S )-[4-(2-methylbutyloxycarbonyl)phenyl] 4-n-heptylbiphenylcarboxylate (MBHB) in its smectic A (SmA), unwound chiral smectic C (uSmC*) and chiral smectic C (SmC*) phases has been studied by means of ²H NMR spectroscopy. Zeeman (T _1Z) and quadrupolar (T _1Q) spin-lattice relaxation times have been analysed to extract dynamic parameters (diffusion coefficients and activation energies). The small step rotation diffusion model in the uniaxial approximation has been used to describe overall spinning and tumbling motions, and the strong collision model to describe the internal reorientations of the aromatic fragment. Relaxation data in the SmC* phase have been analysed by using a theoretical approach. The dynamic features obtained in the smectic phases of this mesogen are here presented and discussed in comparison with the results obtained in other ferroelectric liquid crystals, focusing on the fast regime of motions.     

Circular dichroism in ferroelectric and antiferroelectric liquid crystals

Liquid crystal induced circular dichroism (LCICD) measurements were made to investigate the pretransitional phenomena in the S_A phase just above the S_A-S*_C and S_A-S*_CA phase transitions of both the first and the second order. The pretransitional LCICD in S_A was observed in the second order phase transition to S*_c and S*_cA, suggesting the existence of a dynamic helical structure in S_A. Such behaviour disappears when the transition is of the first order. It seems that the handedness of the dynamic helix in S_A is the same as that in S*_c even when the lower temperature phase is S*_cA. This is explained as a result of a dominant contribution of ferroelectric soft mode.     

Observing molecular motions of a fluorinated ferroelectric liquid crystal using fluorine-19 NMR

Fluorine-19 NMR spectra have been recorded for both the phases (cholesteric and smectic C*) of the ferroelectric liquid crystal 2,3-difluoro-4-octyloxybiphenyl-4'(-4'-oxycarbonyl-(S)-1-chloro-3-methylbutyl)benzoate, using a conventional high-resolution NMR probe. The magnitude of the dipolar coupling constant, D_FF0, was found to increase with decreasing temperature in the cholesteric phase with a sudden increase (of approximately 1 000 Hz) at the cholesteric-smectic C* transition, whereafter a more gradual change was observed. The order parameter, was S_FF, subsequently calculated for each temperature.